My Apartment Threw Out $20k Worth Of My Belongings From My Storage.

A storage unit came available in my apartment complex a few months back and my roommate and I called our property manager to get access the storage. He agreed and turned our key fobs on that allowed for us to go to the storage area of the complex. We moved our things in a few weeks later, no complaints. This past month, the apartment complex said everyone needs to claim their storage unit by Feb. 1st. I called downstairs to claim our unit that same morning. Leasing wasn't available but our front desk staff said she got it and will let them know. A week later, I went down to my storage as usual, but this time, my key fob wasn't working. I then contacted management and long story short, they said they never gave me a storage unit at all. They implied that my fob somehow illegally gained access to the storage unit, but all of our items are now gone. The manager said to give him receipts of our lost items, and we sent them over. We were then told that there was no way we could have had almost $20K worth of items in there because they looked old, etc. Although the receipts show purchase dates from years prior (ofc they'll look old and worn!). The pricier items, he conveniently said they weren't there after "looking through" our items. He contacted their legal dept, as this is a corporate landlord and they said since I didn't sign a contract for the unit, there's nothing they can do. I reviewed my lease and saw that we signed a storage addendum clause. It doesn't show anywhere on my lease that a new addendum with the unit number would need to be signed to have legal rights to the unit. We were under the impression that once the storage addendum was signed in the original lease, all the property management had to do was add it to our account. There was no way for me to know this, again, it's not stated in the lease what their policy is on gaining legal access. I filed a renters insurance claim but I am concerned it may get denied as it is more of a civil case, instead of the standard theft/vandalism claim. Do I have grounds to sue? or I can't because I didn't sign a revised addendum?